1. Field of the Invention
The present invention relates to silicon nitride sintered bodies having particularly excellent strength and creep resistance at high temperature and a method for producing the same.
2. Description of the Prior Art
In general, silicon nitride sintered bodies have superior mechanical properties at relatively high temperatures as compared to metals, so that these bodies have generated particular interest as structural materials for gas turbine parts, adiabatic engine parts, ball bearings for high temperature, parts for various high temperature furnaces and the like, which operate at high temperatures.
For the production of silicon nitride sintered bodies, the following processes have been used.
For example, to silicon nitride powder is added a sintering aid selected from the group consisting of MgO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, CeO.sub.2 and the like and the resulting mixture is sintered under an ambient or a high pressure non-oxidizing atmosphere.
O. de Pous discloses in U.S. Pat. No. 4,264,547 a method for improving the strength at high temperature of a silicon nitride sintered body containing Al.sub.2 O.sub.3 and MgO in a composition shown in FIG. 2 of this specification by decreasing the amount of its grain boundary phase.
A shaped body of Si powder is fired under nitrogen gas (reaction bonding process).
Alternatively, Si powder is mixed with a sintering aid, such as MgO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 and the like and the resulting mixture is formed into a shaped body and then the shaped body is nitrided in nitrogen gas, after which the shaped body is additionally sintered at a high temperature under an ambient or a high pressure non-oxidizing atmosphere.
However, among the prior silicon nitride sintered bodies, when the silicon nitride sintered body parts obtained by the sintering process under ambient pressure, are used at a high temperature of about 1,400.degree. C., such parts have a very poor durability because of the deficiency of the strength at high temperature. The sintered bodies obtained by sintering under high pressure, that is a hot press process or a hot isostatic press process, are very difficult to produce and have excessive manufacturing cost in the production of large articles, complicated shapes or mass production of articles.